1. Field of the Invention
The present invention relates to a projector optical system for projecting illumination light modulated by a light valve onto a screen under magnification; and, more specifically, to a projector optical system comprising a digital micromirror device mounted with a number of mirror elements with variable light-reflecting directions capable of changing the direction of reflection of irradiation light in response to video signals, so that only signal light can be reflected toward a projection optical system, and a projector apparatus using the same.
2. Description of the Prior Art
Recently, the market of projectors has been expanding because of their widespread use in presentations and needs for seeing pictures from DVD players and the like on larger screens. In particular, as the demand for high-definition images has been increasing, projector optical systems using micromirror devices (hereinafter referred to as “DMD”) have been developed as a technique which drastically increases the number of pixels without enhancing the size of optical systems.
When making presentations by employing personal computers, projector apparatus are often used in bright spaces. Hence, there have been demands for developing projector apparatus which can yield sufficiently bright pictures even in such an environment.
For brightening an image projected from a projector apparatus, the output of a light source used therefor may be raised. However, raising the output of the light source not only increases the manufacturing cost but also requires measures against the heat generated from the light source, and shortens the life of the light source.
On the other hand, the effective F number of illumination optical systems may be made smaller, so as to carry out illumination efficiently, thereby brightening the image projected from the projector apparatus.
As such a projector optical system, examples of conventional projector optical systems using DMD will be explained with reference to drawings. FIG. 6 is a view showing the configuration of a major part of a projector optical system for separating an illumination luminous flux and a projection luminous flux from each other by using a total reflection prism. FIG. 7 is a sectional view showing the configuration of a projector optical system for separating an illumination luminous flux and a projection luminous flux from each other at a pupil position of a projection optical system.
In the conventional projector optical system for separating an illumination luminous flux and a projection luminous flux from each other by using a total reflection prism, as shown in FIG. 6, illumination light emitted from a light source (not depicted) is totally reflected by a total reflection surface 91 of a prism 90, so as to be made incident on a DMD 70. Then, the illumination light modulated by the DMD 70 enters a projection optical system (not depicted), so as to be projected onto a screen (not depicted). A cover glass sheet 71 is attached to the surface of the DMD 70.
As shown in FIG. 7, the conventional projector optical system for separating an illumination luminous flux and a projection luminous flux from each other at a pupil position of a projection optical system comprises a light source unit 10; an integrator optical system 20 constituted by a rod integrator 201 for homogenizing a luminous flux from the light source unit 10 and a pair of lenses 202, 203; a mirror 403; a pair of condenser lenses 401, 402; a DMD 70; and a projection optical system 80 including a plurality of lenses. A cover glass sheet 71 is attached to the surface of the DMD 70.
In this projector optical system, the condenser lens 401 is disposed near the pupil position of the projection optical system 80. The illumination luminous flux passes through the condenser lens 401 disposed near the pupil position whereas the projection luminous flux from the DMD 70 enters the inside of a frame (not depicted) of the projection optical system 80, whereby these luminous fluxes are separated from each other. Namely, the illumination light emitted from the light source unit 10 so as to become substantially parallel light is turned into a uniform luminous flux by the integrator optical system 20, which is reflected by the mirror 403 and then is collected by the condenser lenses 401, 402, so as to be made incident on the DMD 70. The illumination light modulated by the DMD 70 so as to carry image information enters the projection optical system 80 again by way of the field lens 402, so as to be projected onto a screen (not depicted).
However, in the projector optical systems using DMD, such as the optical system for separating the illumination luminous flux and the projection luminous flux from each other by using the total reflection prism and the projector optical system for separating the illumination luminous flux and the projection luminous flux from each other at the pupil position of the projection optical system, the effective F number and the rotation angle of a minute mirror element constituting the DMD have a close relationship therebetween, whereby the effective F number of the illumination optical system cannot be made smaller unless the angle of rotation of minute mirror elements constituting the DMD is changed. Also, the angle of rotation of minute mirror elements constituting the DMD is determined according to predetermined standards and thus cannot be changed easily.
The total reflection angle (indicated by A in FIG. 6) in the total reflection prism is determined by the total reflection condition and cannot be changed arbitrarily.